The present invention relates to wireless communications, and in particular to a fully differential front end receiver architecture providing broadband performance.
Receiver front ends for wireless communication systems typically require a low noise amplifier (LNA) followed by a double-balanced mixer to pre-amplify incoming signals and subsequently down-convert those signals to an appropriate intermediate frequency (IF). Due to ever-present competitive concerns, there is a need to provide high performance and competitive pricing. Product cost is directly proportional to the number of components necessary to implement a design, and given the need to provide circuitry capable of performing over various frequency bands and in different modes, the ability to provide architectures capable of supporting multiple frequency bands without requiring redundant circuit elements for each band significantly reduces overall cost.
Unfortunately, providing architectures capable of performing over multiple frequency bands often requires more expensive components to maintain performance levels over a broad frequency range. Further, operation over multiple modes often subjects receiver circuitry to varying input signal levels, which often results in poor performance or distortion. Previous mixing circuits suffer from poor second-order intermodulation performance, and are unable to handle significant fluctuations in signal levels associated with the incoming signal.
Accordingly, there is a need for a broadband receiver front end capable of being implemented in a cost-effective manner while achieving high levels of performance. There is a further need for such a receiver capable of handling significant fluctuations in signal levels associated with the received signals.
The present invention provides a fully differential receiver front end including a differential low noise amplifier and a transformer-based mixer for down-converting a radio frequency (RF) signal to an appropriate intermediate frequency (IF) signal or baseband signal in a direct conversion system. The receiver front end has exceptional second-order intermodulation performance, and may include circuitry providing a reduced gain in order to handle incoming signals having high signal levels. The receiver front end is very broadband and is able to cover multiple communication modes or bands using a common switching core in the mixer. The invention is also applicable to quadrature-based receiver front ends.
In one embodiment, a differential RF input signal drives the plus and minus terminals of a center tapped primary winding, which couples the differential RF input signal to the plus and minus terminals of a center tapped secondary winding. From the secondary winding, the coupled, differential RF signal drives a double-balanced mixer core operating in response to a differential local oscillator signal. The output of the mixer core is a differential IF signal representative of the differential RF input signal down-converted at the local oscillator frequency.
A step reduction in gain associated with the RF input signal may be implemented using bypass circuitry coupled across the primary of the transformer, wherein a relatively small resistance or impedance is coupled across the primary to effectively reduce the load of the differential amplifiers driving the primary. Preferably, the bypass circuitry is implemented such that both windings of the primary are treated equivalently. In one embodiment, opposingly configured PMOS or NMOS transistors are used to implement the bypass circuitry, such that when the opposing transistors are turned on, the effective channel resistance is applied across a primary of the transformer. In an alternative embodiment, a symmetrically structured multi-fingered MOS device may be used as or in association with the bypass circuitry to provide a reduced gain when the RF input signal is at a high level. The MOS device can be realized in either NMOS or PMOS.
In quadrature-based systems, an additional quadrature mixer may be implemented by placing the primary of a second transformer in parallel with the primary of the first transformer to derive the quadrature phase IF signals. Preferably, any step gain control is provided simultaneously to the primaries of both transformers, such that the in-phase and quadrature phase IF signals are treated consistently.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.